While there are many ways to weave emotion into music, two of the simplest are tempo and key. Happy tunes mostly have fast tempos and major keys. Sad songs often have slow tempos and minor keys.

To prove this, University of Toronto professor Glenn Schellenberg set up an experiment. He asked a graduate student to identify Top 40 songs that match these criteria. Finding happy songs from the 1960s and '70s was easy – The Beatles' "She Loves You" has a fast tempo and a major key. But with each successive decade, the hunt turned up fewer and fewer examples.

Looking at more than 1,000 Billboard hits since 1965, his team found that the average song has become longer, slower, and less happy-sounding. The share of major-key songs dropped from 85 percent of pop hits in the 1960s to just 42 percent in the 2000s.

What happened? Schellenberg says that pop culture seems to have developed an aversion to saccharin-sounding music. Many popular songs now mismatch tempo and key. By pairing mixed emotional cues, he says, artists create a more complex sound.

To the modern ear, "if you have something that sounds unambiguously happy, it kinda sounds childish," he says. Schellenberg points to Aqua's 1997 dance hit "Barbie Girl." The fast-paced, major-key song sold well – but as a guilty pleasure. More critically accepted dance songs such as Kylie Minogue's "Can't Get You Out of My Head" and Madonna's "Hung Up" share the same quick beat, but both are in a minor key.

Even this summer's sugary hit "Call Me Maybe," by Carly Rae Jepsen, tosses in a few minor chords.

Schellenberg says that classical music went through a similar transition. By the 1800s, Romantic-era compositions shook up traditional tempo/key pairings. This shift in classical music transpired over 300 years. Pop music rounded the same bend after only 50 years.

A previous study by Schellenberg found that trained musicians prefer songs with nontraditional tempo/key pairings. Mixed music, he says, sounds "sophisticated." Now, it seems that pop culture at large favors this new "sophisticated" sound.